The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a user equipment (UE) 150 is wirelessly connected to a radio base station (RBS) 110a-c commonly referred to as an eNodeB (eNB), as illustrated in FIG. 1a. The eNBs 110a-c are directly connected to the core network (CN) 190 via the S1 interface. In UTRAN however, the radio base stations or NodeBs (NB) are connected to the CN via a Radio Network Controller (RNC) which controls the NBs connected to it.
In the first release of the E-UTRAN, network-scheduled resources are used for all uplink data transmissions. A UE that wants to initiate uplink transmission must first request uplink resources by transmitting a scheduling request, either in a random access procedure or through a dedicated scheduling request. The eNB will select the resource blocks to be assigned to the user and will return a scheduling grant indicating the selected resources. Once the UE has received the scheduling grant it can start uplink data transmission on the assigned uplink resources. This procedure causes an extra delay for uplink data packets in cases when the UE is not currently scheduled at the time the data becomes available. The delay is due to that the UE has to request a scheduling from the network and wait for the scheduling grant before being able to transmit the data.
To reduce the delay it has been proposed in 3GPP to also introduce a contention-based (CB) uplink transmission scheme. CB transmission schemes can provide good performance when the load is low, although the risk of collisions may lower the performance at a high load. One scheme that has been proposed is that the eNB repeatedly transmits uplink scheduling grants, so called CB grants, that are similar to normal scheduling grants, with the exception that a CB grant is applicable to multiple UEs and not only to one as in the case of a normal scheduling grant. A UE that needs to transmit uplink data when not being scheduled, may thus use such a CB grant and its assigned resources for uplink transmission. Between the CB grant and the actual uplink transmission, there is a fixed time delay of 4 ms or more. A collision may occur when several UEs use the same CB grant for their uplink transmission, which will typically result in a reception error as the eNB will not be able to decode data transmitted by several UEs in a same resource. By choosing whether to transmit a CB grant or not, the base station can decide for each sub frame which uplink resources that may be used for regular scheduled transmissions and which ones that may be used for contention-based access.
To reduce the UE power consumption, a UE is normally configured to shut down its receiver when there is little or no activity. This feature is called discontinuous reception (DRX). Likewise, to reduce the base station power consumption and decrease interference, the eNB may be configured to turn off its transmission when there is little or no activity, which is referred to as discontinuous transmission (DTX).
However, a UE that want to use CB uplink transmissions will also need to use DRX in order to save power consumption, which will prevent the UE from continuously receiving the CB grants from the eNB. Therefore, when new data or data packets arrive for uplink transmission, the UE will typically not have received any CB grants recently. The UE will thus have to wait for the next CB grant to be received and processed before performing its uplink transmission, which will thus cause an unwanted delay for accessing the uplink resources, in addition to the fixed delay between grant and transmission of 4 ms.
Furthermore, the proposed scheme requires that the eNB repeatedly transmits CB grants. One problem with this scheme is thus that downlink transmission resources will be used for CB grants that in many cases will not result in any transmission, thus wasting transmission capacity, and creating unnecessary interference. Another problem is that the transmission of CB grants may hinder the usage of DTX in a situation with little or no other activity, which will thereby increase the eNB power consumption and the interference.